1. Field of the Invention
The present invention relates to a controller for a fixed disk drive and more particularly to a fixed disk drive controller for use in a word processing system.
2. Description of the Prior Art
It is desirable in word processing equipment to have a fixed disk drive to store data and programs.
Many word processing systems currently available consist of a keyboard, a printer, a display and a floppy disk drive, all controlled by one or more microprocessors. Floppy disk drives have been shown to be cost effective means by which programs and data can be stored and accessed on demand by the operator. Compared to other forms of data storage, such as magnetic tape cassettes, magnetic cards and punched paper tape, floppy disks allow the user to access more information and to access the information faster.
It has been found, however, that for larger systems requiring more data manipulation and formatting, the floppy disk system is inadequate. Resource sharing and time sharing data or word processing systems, for example, generally require a larger data base and shorter access time than do stand alone systems. Moreover, sophisticated operations such as merging of files, sorting of data within files, and dictionary capabilities often require more memory space and shorter cycle times to be performed efficiently in real time. Finally, for complex data handling procedures, increased reliability is also required.
The relative advantages of floppy disks over magnetic tape, cards and paper tape unfortunately dissolve in the aforementioned environments. The solution to the problems inherent in floppy disk systems can be found by using one or more hard or fixed disks in conjunction with or in lieu of floppy disks.
As word processing systems become more sophisticated, the storage capacity of their mass memory mediums must increase and the speed at which data is stored and retrieved must decrease. For these reasons, one or more flexible (floppy) disks may not be adequate to access a great amount of data in a relatively short time, even if the floppy disks are dual-sided, double density.
A rigid or fixed disk, on the other hand, has a larger storage capacity and the disk drive associated with it is generally capable of rotating it at a greater velocity. It is therefore advantageous to adapt such a rigid disk for use with a word processing system. A rigid disk, and in particular a Winchester disk, is coated on both sides with a magnetic medium, so that two surfaces per disk are available for the storage of data. Each Winchester read/write head has three rails, or raised surfaces. The trailing end of the middle rail holds a magnetic core with wire coiled around for writing and reading the data. The two outer rails govern the flow of air. The force that results is sufficient to support a weight of 10 grams at a height of half a micrometer above the disk. The disks and the head assemblies in such a memory are sealed in a small chamber in which the air is continuously recirculated an filtered to exclude any dust particles larger than 0.3 micrometer in diameter.
The quantity of data that can be stored on a disk depends on how much of its surface area is magnetized for the storage of a bit. The width of a magnetized region, or equivalently the width of a data track, is affected by limitations on both the head and the disk. The width of the center rail of a Winchester head is approximately 20 micrometers, which corresponds to a track density on the disk of about 1,000 per inch of the radius. On a floppy disk however, the track density is only 48 tracks per inch for single density disks (96 tracks per inch for double density disks).
In high-performance memories the one of the surfaces of a disk is devoted to patterns of bits that continuously yield information on the position of the head. Any deviation from the proper position causes the generation of a signal in the head that actuates a motor for repositioning. Such patterns of bits may be embedded within the stored data itself.
The number of bits that can be written along a track also is affected by limitations on both the head and the disk. As a result of all these constraints the number of reversals in magnetism along a data track in a device that records digital data by magnetic saturation and employs a Winchester head is about 10,000 per inch. The quantity of data stored ranges from about 20 million bits for one surface of a floppy disk to billions of bits for high-performance rigid disks.
The rate at which bits are written or read along a track is called the data rate. It ranges from hundreds of thousands of bits per second for floppy disk systems to 10 million bits per second for rigid disk systems. The main reason for the difference is the fact that floppy disks must rotate at lower speeds.
The configuration of disk drives and their associated controllers has heretofore been a matter of connecting a CPU or processor to one or more disk controllers and connecting one disk drive to each of the disk controllers. A more elaborate system of daisy chaining disk drives, one of which drives is connected to a disk interface and controller, is disclosed in U.S. Pat. No. 4,064,561, issued to Jennings. In that system, two disk interface and controller units are shown connected to a CPU. Each of the disk interface and controller units controls up to four daisy chained disk drives, each of which drives includes two disks, one fixed and the other removable. Thus, a maximum of 16 disks can be incorporated in that system.
A system in which a fixed disk controller is connected to a floppy, or removable disk controller provides a number of advantages in terms of system efficiency and flexibility. One of the advantages is that the system can be configured to operate satisfactorily in a minimum configuration of a single floppy disk, but yet can be expanded to include one or more fixed disks in addition to or in lieu of the floppy disk. Thus, the advantage of such a piggy back system of fixed disk control over the aforementioned daisy chained disk configuration is that one of the fixed disk drives can be removed in the piggy back system with minimal effort without significantly affecting system operation. A removal of one of the daisy chained disk drives, on the other hand, obviously renders all system drives connected to it down-stream inoperable.